1. Field of the Invention
The invention relates to a steering control apparatus intended to operate a steering system including a steering operation actuator that steers steered wheels of a vehicle by displacing a rack shaft in an axial direction relative to a regulation member fixed to a vehicle body to regulate displacement of the rack shaft in a direction crossing an axial direction while permitting relative displacement of the rack shaft in the axial direction.
2. Description of the Related Art
For example, Japanese Patent Application Publication No. 2009-236297 (JP 2009-236297 A) proposes a steering gearbox that is fixed to a vehicle body utilizing a rubber mount bush and that regulates displacement of a rack shaft in a direction crossing an axial direction while permitting relative displacement of the rack shaft in the axial direction. Specifically, JP 2009-236297 A discloses that the steering gearbox is elastically supported on the vehicle body by forming mounting holes in the steering gearbox at a plurality of positions, fitting mount bushes in the mounting holes, inserting bolts into the mount bushes, and fastening the bolts to the vehicle body. Regulation members may be defined as those of the steering gearboxes which are structured to combine with an element that regulates displacement of the rack shaft in the direction crossing the axial direction while permitting relative displacement of the rack shaft in the axial direction.
The steering system steers steered wheels of the vehicle by displacing the rack shaft relative to the steering gearbox in the axial direction. Thus, a steering control apparatus intended to operate the above-described steering system by performing steering operation control on the steered wheels can control the steered angle of the steered wheels by controlling the relative displacement of the rack shaft relative to the steering gearbox in the axial direction. However, when the steering gearbox is elastically supported on the vehicle body using the mount bushes as described above, the steering gearbox itself may be displaced relative to the vehicle body in the axial direction due to elasticity of the mount bushes. In this case, a difference equal to the displacement of the steering gearbox relative to the vehicle body is present between the relative displacement of the rack shaft relative to the steering gearbox in the axial direction and the relative displacement of the rack shaft relative to the vehicle body in the axial direction. The difference may act as an error in control of the steered angle of the steered wheels when the steered angle is controlled by controlling the relative displacement of the rack shaft relative to the steering gearbox in the axial direction.
On the other hand, when mounted to the vehicle body, the steering gearbox may be firmly fixed directly to the vehicle body without using the rubber mount bushes. In that case, an error in control of the steered angle of the steered wheels can be suppressed even when the steered angle is controlled by controlling the relative displacement of the rack shaft relative to the steering gearbox. However, in that case, users' needs are not always met; when the steered wheels are steered through a large angle, impact is likely to be transmitted to the vehicle. Consequently, a phenomenon resulting from execution of steering operation control on the steered wheels is significantly affected by a technique for fixing the regulation member in the steering gearbox to the vehicle body.